Sorry to hear you’re having troubles!
All Particle SIM cards should be able to connect in Mexico without any configuration beyond the typical setup.
Would you be able to provide me with the device IDs at our support portal or directly to me via private message here on the forums?
In addition, I encourage you to check AT&T’s coverage maps here to ensure that there is appropriate coverage in your area.
Depending on coverage in your area, it may be necessary to move your device either near a window or outside in order to establish a connection. Also ensure that you are using the provided u.FL Taoglas flex antenna that came with your device and that it is connected to the proper port. There are u.FL connectors for cellular, mesh, and bluetooth. Connecting the antenna to the wrong one will cause there to be no connectivity.
Once I have the device IDs, I can check in our systems to ensure that everything is active. If the devices are active and we are still having troubles, we can circle back here for collecting serial logs from the device.